LCDs are widely used in various modern information products, such as notebooks, personal digital assistants (PDAs), video cameras and the like. Because liquid crystal in an LCD does not emit any light itself, a backlight system is usually needed to enable the LCD to display images.
A typical backlight system includes a plurality of backlight lamps, and a backlight control circuit. The backlight control circuit is used for feeding back currents of the backlight lamps, and protecting the backlight system when an open circuit occurs in any of the backlight lamps.
Referring to FIG. 3, one such backlight control circuit 100 includes a pulse width modulation integrated circuit (PWM IC) 110, an inverter circuit 130, a backlight lamp unit 150, a first feedback circuit 140, a second feedback circuit 160, and a protecting circuit 170.
The backlight lamp unit 150 includes a first lamp 151 and a second lamp 152. The first lamp 151 and the second lamp 152 both have a positive end and a negative end. The PWM IC 110 includes a signal output terminal 111, a current feedback terminal 113, a protecting output terminal 115, and a voltage feedback terminal 116. The signal output terminal 111 is connected to the inverter circuit 130. The voltage feedback terminal 116 is connected to the first feedback circuit 140. The current feedback terminal 115 is connected to the second feedback circuit 160. The protecting output terminal 115 is connected to the protecting circuit 170.
The inverter circuit 130 includes a signal input 131, a first driving terminal 132, and a second driving terminal 133. The first driving terminal 132 and the second driving terminal 133 output an AC voltage to the positive ends of the lamps 151, 152 respectively. A value of the AC voltage can be 1500V. The AC voltage at the first driving terminal 132 has a phase opposite to that at the second driving terminal 133.
The first feedback circuit 140 includes two high voltage feedback inputs 141 and a high voltage feedback output 142. The two high voltage feedback inputs 141 are connected to the positive ends of the lamps 151, 152 respectively. The high voltage feedback output 142 is connected to the voltage feedback terminal 116 of the PWM IC 110. The first feedback circuit 140 outputs a first feedback signal to the voltage feedback terminal 116.
The second feedback circuit 160 includes a current input 161 and a low-voltage feedback output 162. The current input 161 is connected to the negative ends of the lamps 151, 152. The low-voltage feedback output 162 is connected to the current feedback terminal 113 of the PWM IC 110. The second feedback circuit 160 outputs a second feedback signal to the PWM IC 110 corresponding to the current at the negative ends of the lamps 151, 152.
The protecting circuit 170 includes a first resistor 171 and a capacitor 172. One end of the first resistor 171 is coupled with the protecting output terminal 115 of the PWM IC 110, and the other end of the first resistor 171 is grounded via the capacitor 172. The first resistor 171 is used for controlling the charging time of the capacitor 172.
When an open circuit occurs in any of the lamps 151, 152, the current input 161 feeds back the current of the lamps 151, 152, and the second feedback circuit 160 outputs a lower second signal to the PWM IC 110. When the second signal is lower than a first reference voltage, the PWM IC 110 outputs a pulse-time ratio signal to increase the working voltage of the backlight unit 150 through the inverter circuit 130. At the same time, the first feedback circuit 140 outputs the first signal to the PWM IC 110. The PWM IC 110 compares the first signal with a second reference voltage. When the first signal is higher than the second reference voltage, the PWM IC 110 outputs a signal to charge the capacitor 172 via the protecting output terminal 115. When the voltage of the capacitor 172 reaches a predetermined potential, for example 3V, the PWM IC 110 stops the inverter circuit 130 from driving the backlight lamp unit 150, so as to protect the backlight lamp unit 150.
As described above, the inverter circuit 130 stops the backlight lamp unit 150 after a period of time has elapsed from the time when the PWM IC 110 outputs the signal to charge the capacitor 172. During this period, the PWM IC 110 continuously increases the voltage difference between the lamps 151, 152. The voltage difference between the lamps 151, 152 may increase and induce a spark discharge. The spark discharge is liable to destroy the backlight lamp unit 150. Thus, the backlight control circuit 100 has low reliability.
It is, therefore, desired to provide a backlight control circuit that can overcome the above-described deficiencies.